Molecular design of conjugated polymers for efficient electroluminescence (EL) and color tuning has long been one of the most important subjects in the development of polymer light emitting diodes (PLED) and can be carried out in two ways: by chemical and physical methods. The chemical method, involving the incorporation of charge-transport moieties on the main chain (Wu, F. I., et al., Macromolecules, 38, 9028 (2005). Kim, J. K., et al., J. Mater. Chem., 91, 2171 (1999). Liu, M. S., et al., Chem. Mater., 13, 3820 (2001)), flexible side chain (Ego, C., et al., Adv. Mater., 14, 809 (2002). Chen, X., et al., J. Am. Chem. Soc., 125, 636 (2003). Shu, C. F., et al., Macromolecules, 36, 6698 (2003)), and chain ends (Miteva, T., et al., Adv. Mater., 13, 565 (2001)), has been extensively studied for poly(phenylene vinylene)s, polyfluorenes, and other polyarylenes in order to promote balanced hole and electron fluxes and to adjust highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, as well as the band gap for color tuning. Taking polyfluorenes as an example, incorporation of triphenylamine in the main chain and oxadiazole in the side chain provides an improvement in the efficiency and purity of blue emission to 2.07 cd A−1 and a Commission Internationale de l'Eclairage (CIE) value of x+y=0.29, respectively, which is the best blue fluorescence device that has been reported so far (Wu, F. I., et al., Macromolecules, 38, 9028 (2005)). However, chemical methods require elaborate synthesis. Physical methods include blending a conjugated polymer with dopants (Huang, Y., et al., Mater. Chem. Phys., 93, 95 (2005). Zhang, X., et al., Chem. Phys. Lett., 422, 386 (2006). Zhang, Y, et al., Appl. Phys. Lett., 85, 5170 (2004)), tuning a chain conformation (Chen, S. H., et al., Macromolecules, 37, 6833 (2004). Chen, S. H., et al., Macromolecules, 38, 379 (2005). Chen, S. H., et al., J. Phys. Chem. B, 109, 10067 (2005). Ariu, M., et al., Synth. Met., 111-112, 607 (2000)), and manipulating a supramolecular structure (Apperloo, J. J., et al., Macromolecules, 33, 7038 (2000)). The former involves energy transfer and charge trapping allowing an enhancement of device performance in addition to color tuning and has been studied extensively. Studies on the effects of the tuning of chain conformation on EL are scarce, but studies on the effect of the manipulation of the supramolecular structure on the photoluminescence (PL) of the blue-emitting polymer poly(9,9-di-n-octyl-2,7-fluorene) (PFO) are extensive.
Because of its highly coplanar backbone, PFO can be physically transformed by into a variety of supramolecular structures (Chen, S. H., et al., Macromolecules, 37, 6833 (2004). Chen, S. H., et al., Macromolecules, 38, 379 (2005). Chen, S. H., et al., J. Phys. Chem. B, 109, 10067 (2005). Ariu, M., et al., Synth. Met., 111-112, 607 (2000)), such as crystalline phases (i.e., α and α′ phase) and noncrystalline phases (such as amorphous, nematic, and β phase, which has an extended conjugation length of about 30 repeat units, as evidenced by wide-angle X-ray diffraction (Grell, M., et. al., Macromolecules, 32, 5810 (1999))). Among these structures, β phase has attracted the most attention because of its specific physical properties, such as a lower extent of triplet exciton formation (Hayer, A., et. al., Phys. Rev. B, 71, 241302 (2005)), a reduced ability to be photobleached on the single-molecule scale (Becker, K., et. al., J. Am. Chem. Soc., 127, 7306 (2005)), and efficient energy transfer from the amorphous to the β phase (Khan, A. L. T., et al., Phys. Rev. B, 69, 085201 (2004)). β phase can be physically formed by dissolving PFO in solvents with lower solvent power and higher boiling points (Khan, A. L. T., et al., Phys. Rev. B, 69, 085201 (2004)) or in a solvent/nonsolvent mixture (for example, chloroform/methanol) (Scherf, U., et al., Adv. Mater., 14, 477 (2002)), by exposing a PFO film to solvent vapors (i.e., hexane, cyclohexane, tetrahydrofuran, or toluene) (Grell, M., et. al., Macromolecules, 32, 5810 (1999)), or by applying specific thermal treatment to a PFO film (cooling and reheating to room temperature) (Grell, M., et. al., Macromolecules, 32, 5810 (1999)). In our previous work (Hung, M. C., et al., J. Am. Chem. Soc., 127, 14576 (2005)), we reported that the use of an electron-deficient moiety (such as triazole) as an end-capper for PFO can induce a trace amount of β phase without any further physical treatment and this can be taken as a quasiphysical approach for β-phase formation. Very recently, PFO with a so-called intrinsically doped β phase has been demonstrated to be a potential material for an electrically pumped laser (Rothe, C., et al., Adv. Mater. 18, 2137 (2006)). However, the effect on device efficiency in a presence of the β phase has not been explored, probably because of complicated and time-consuming procedures for tuning β-phase content.